1. Industrial Field of the Invention
The present invention relates to improvements of tool rests and a tailstock of a machine tool for complex machining which mainly comprises two tool rests one of which performs usual circular lathe machining whereas the other tool rest exclusively performs non-circular machining.
2. Prior Art
In mechanical machining of metal, generally, each workpiece must be subjected to various kinds of machining to finish it as a product. In the case of a workpiece which requires non-circular machining, circular lathe machining, milling and so forth, it is necessary to process the workpiece by machine tools having the respective machining characteristics in different processing steps, i.e., to conduct non-circular machining by a machine tool of a copying method or numerical control method (U.S. Pat. No. 5,085,109), circular machining by a lathe (G.B. Patent No. 2,178,991A), and milling by a machining center or a milling machine (U.S. Pat. No. 4,358,888). Further, every time the machine tools are changed for the next processing step, a skillful operator uses jigs and measuring devices so as to obtain alignment of a reference position for the previous step and a reference position for the subsequent step.
A workpiece is supported by a chuck at a distal end portion of a main spindle which serves as workpiece supporting means, and by thrust of a tailstock spindle of a tailstock. In this case, clamping force of the chuck and thrust of the tailstock spindle are determined by hydraulic pressure control such that the supporting force during machining of the workpiece can endure the maximum cutting resistance. Usually, the workpiece supporting force is unchanged until the machining is over. The clamping force and the thrust more than necessary cause the workpiece to be deformed or distorted, and it is difficult to maintain high machining accuracy.
When the machining arrangement is changed, a tailstock is manually moved and positioned in most cases. In the case of automatic operation, the tailstock is moved for a predetermined distance forwardly or backwardly by means of a hydraulic cylinder or the like, and the tailstock can not be freely positioned at an intermediate location in this stroke.
As described above, various kinds of machining are often required for a single workpiece. Conventionally, there has been used a machine tool for complex machining which performs lathe machining, milling and drilling by use of C-axis, Z-axis, X-axis and Y-axis mechanisms. However, there has not been found a machine tool for complex machining which enables non-circular machining such as elliptic machining.
It is an object of the present invention to provide a machine tool for complex machining which has a non-circular machining function, a circular lathe machining function, and/or a milling function, in order to improve accuracy in workpiece machining including non-circular machining and to lessen the number of processing steps, thereby enhancing the productivity. For instance, a piston for an engine has pin holes, oil holes and ring grooves, and the outer periphery generally has an elliptic cross-sectional configuration whose length and breadth have a slight difference. In order to process such a workpiece, elliptic finishing requires an elliptic machining function, and machining of the ring grooves requires a lathe machining function, while machining of the pin holes and oil holes requires a drilling function. Especially when the elliptic finishing is performed, the target portion of the workpiece is usually processed into a circular cross-sectional configuration before the final elliptic finishing so as to improve the machining accuracy. Also, alignment of machining references such as positional relations between the pin holes and the oil holes with respect to the length and breadth positions of the elliptic cross-sectional configuration is necessary for finishing the piston as a product.
Consequently, alignment of the reference of non-circular machining and the reference of milling is required. In the prior art, every time the machining arrangement is changed for the next processing step, machining reference positions must be determined by use of precise jigs and various measuring devices so as to determine reference positions for the preliminary machining. Such operations interposed between the processing steps often induce errors, and accumulated errors become so large that it is difficult to maintain the machining accuracy of each workpiece at a certain level when the workpiece is finished. Taking this into account, a machine tool for complex machining which has a non-circular machining function, a circular lathe machining function, and/or a milling function is required for improving accuracy in workpiece machining including non-circular machining and for enhancing the machining efficiency.
As workpiece supporting means, there have conventionally been employed a method of supporting a workpiece only by a chuck at a distal end portion of the main spindle, and a method of supporting a workpiece both by a chuck at a distal end portion of the main spindle and a tailstock spindle on the side of a tailstock (as disclosed in, for instance, G.B. Patent No. 2,178,991A). In general, considering the cutting resistance at the time of workpiece machining, clamping force of the chuck and thrust of the tailstock spindle are determined by hydraulic pressure control, and the workpiece supporting force is kept constant until the machining is over. When the supporting force is constant in this manner irrespective of the cutting resistance of the workpiece, the supporting force exceeds the rigidity of the workpiece, and the workpiece during the machining is deformed or distorted. As a result, when the workpiece is released from the supporting force after the machining is over, it is often found that the workpiece does not have a predetermined machining accuracy as a final product. The elliptic configuration of a piston only allows an error of several microns in the machining accuracy. Therefore, since deformation or distortion caused by the workpiece supporting force greatly influences the machining accuracy, it must be made as small as possible.
Moreover, when the machining arrangement is changed for another workpiece, the tailstock must be moved and positioned in accordance with the size of the workpiece. In this case, manual operation for moving the tailstock requires a large amount of labor, and an operator must be particularly careful for positioning the tailstock precisely.